Crack initiation toughness tests were conducted using the J-integral as a fracture characterizing parameter according to the ASTM Test Method for JIc, a Measure of Fracture Toughness (E 813-87). The specimens were made from a weldment of Type 304 stainless steel (SS) base metal and Type 308 SS weld metal, and were fabricated such that the crack initiation toughness (JIc) of the weld metal could be measured. A peculiar feature of all of the tests was that crack growth occurred in an inverse tunneling mode. The provisional initiation toughnesses (JQ) calculated from the test results were determined to be invalid because the validation requirements on change in crack front curvature and on accuracy of the predicted crack extension were not met. The accuracy of the predicted extensions was improved by applying correction factors to the last measured compliances to account for crack front curvature, plastic deformation, friction, and indentation. The excessive change in crack front curvature was caused by the transition from tunneling growth during fatigue precracking (before the specimens were side grooved) to inverse tunneling during slow stable crack growth. Crack growth by inverse tunneling indicates a constraint condition at the root of the side-grooves that is more severe than in the center. If this is the case, the measured initiation toughness should be a valid indication of the lower bound for high constraint.